As far as marijuana grow operations go, the three warehouses in southwest Denver certainly looked legit.

They were in areas where commercial cultivation operations are common. They had wireless security cameras, build-out plans drawn up by an architecture firm and contracts with a carbon-dioxide supplier. They even employed a security company often hired by legal marijuana businesses.

But, according to search warrants filed in Denver District Court, they lacked one key component: a license.

A worker at one of the grows nabbed by police told detectives, “he did not know if it was licensed, but was pretty sure it was not legal,” according to a search warrant affidavit.

When police raided the warehouses in February and March, they took nearly 1,300 plants.

But Denver authorities have yet to file charges against anyone connected to the warehouses, and a police spokesman said the case is ongoing. Far from an old-fashioned slam-bang marijuana bust, the case has come to show how difficult police say it now is to investigate suspected illegal marijuana growing in Colorado.

Since the passage of limited marijuana legalization in Colorado in 2012, law enforcement officials say many agencies are making fewer busts of illegal marijuana distributors and seizing fewer illegally grown plants, though comprehensive numbers aren’t yet available.

But those same officials contend the drop-off comes not from a decline in illegal growing but from an increased hesitance of detectives to make busts in a state where the margins of legal and illegal cultivation can be blurry. What’s more, if police seize and destroy marijuana later deemed to be legal, they could be sued for damages.

“People are unsure of what they can and can’t do,” Tom Gorman, the director of the Rocky Mountain High Intensity Drug Trafficking Area, said of investigators. “People are shying away. There’s a lot of confusion out there by law enforcement.”

Jim Gerhardt, a sergeant with the North Metro Drug Task Force in Adams and Broomfield counties, said most of the unlicensed grows detectives have busted in his area are in homes. But he said the task force’s number of busts “are way down” this year, something he attributed to pot legalization.

When anyone over 21 can grow up to six plants, it’s tougher for police to unravel whether the plants they find are legal or not.

“There’s a ton of grow operations out there,” Gerhardt said. “There’s a ton of activity in the community. It’s just not as easy to investigate as it was.”